Despite considerable accumulated information on pneumonia, the local inflammatory response to pulmonary infection is not understood. We know neither what happens at the initial site of infection in the first hours and days nor what determines the subsequent course in seemingly normal individuals. While properties of the organisms are surely important, it also seems likely that a proper variety and sequence of host protective reactions is critical. Such reactions might logically include, exudation of complement which, if activated by either the classic or alternate pathway, would act as opsonins of bacteria. Our approach to these questions has been to study pneumonia in rabbits. Pneumonia is produced by percutaneous puncture of the trachea and instillation of Streptococcus pneumoniae (type or XXV), or Pseudomonas seruginosa suspended in hog gastric mucin. Animals will be sacrificed serially so that differences in local responses with time may be determined. Bronchopulmonary secretions will be assayed for opsonic activity and adherence of either complement or immunoglobulins to the surface of bacteria. Since opsonins have not been readily detected, additional variables in the phagocytic systems will be studied. Furthermore, animals deficient in complement components and animals immunized with homologues killed organisms will be studied to further elucidate the role of these factors.